Lubricants such as gear oils, transmission oils, hydraulic oils and greases are required to protect and release heat from internal combustion engines and machine tools, and are also required to meet various properties such as wear resistance, heat resistance, sludge resistance, lubricant consumption characteristics and fuel efficiency. As internal combustion engines and industrial machines which are lubricated have grown in performance and output and have come to be operated under severer conditions in recent years, the lubricant performance that is required is more and more advanced. Recently, in particular, an extension in lubricant life tends to be demanded out of environmental considerations despite the fact that the conditions under which lubricants are used are becoming harsher. This tendency has given rise to a demand for enhancements in heat resistance and oxidation stability, and has further created a demand that the decrease in viscosity due to shear stress caused by engines and machines be reduced, that is, lubricants exhibit enhanced shear stability. On the other hand, in order to enhance the energy conversion efficiency of engines or to ensure good lubrication of engines in an extremely cold environment, importance is placed on temperature viscosity characteristics in which lubricants keep the form of an oil film at high temperatures while still attaining good retention of fluidity at low temperatures. One of the indicators to quantify the temperature viscosity characteristics discussed here is a viscosity index calculated by the method described in JIS K2283. The higher the viscosity index of a lubricant, the more excellent the temperature viscosity characteristics.
As described above, there has been a demand for lubricants having excellent heat resistance, oxidation stability and shear stability and also having good temperature viscosity characteristics.
In particular, lubricants used in automobiles, specifically, automotive gear oils such as differential gear oils and drive oils represented by transmission oils have come to be required to outperform the conventional lubricants in temperature viscosity characteristics and further to exhibit high fluidity at an extremely low temperature such as −40° C., namely, to have excellent low-temperature viscosity characteristics. These viscosity characteristics, which directly affect the fuel efficiency performance of automobiles, are required to be enhanced because after the adoption of the Kyoto Protocol in 1997, governments in the world have recently worked on or have set future targets on controlling carbon dioxide emissions from vehicles and regulating the fuel efficiency.
Based on the governmental decisions, automotive machine parts are more and more compact and receive less lubricants in order to enhance the fuel efficiency so that the fuel efficiency targets will be accomplished. This situation increases the load on lubricants and has given rise to a need for a further increase in lubricant life.
Since automotive gear oils or transmission oils are subjected to shear stress that is applied by gears, metallic belts or the like, molecules used in the lubricant base are broken during use. Consequently, lubricant viscosity reduces. The decrease in lubricant viscosity causes metallic parts in gears to be in contact together, resulting in significant damages to the gears. It is therefore necessary to design the viscosity of a lubricant as produced (the initial viscosity) to be high beforehand in expectation of a viscosity drop during use so that the lubricant after being degraded by use can provide ideal lubrication. SAE (the Society of Automotive Engineers) J306 (automotive gear oil viscosity classification) defines the minimum viscosities after the shear test specified by CRC L-45-T-93 (method C, 20 hours).
As a matter of fact, the life of a lubricant can be increased as the base used in the lubricant has higher shear stability. In this case, the lubricant does not need to be designed with a high initial viscosity and consequently the resistance experienced by gears during stirring of the lubricant can be reduced, which results in an enhancement in fuel efficiency.
Further, good temperature viscosity characteristics, in other words, low dependence of lubricant viscosity on temperature makes an increase in lubricant viscosity small even in a cold environment. Consequently, the increase in gear resistance due to the lubricant is relatively small as compared to conventional levels, and thus the fuel efficiency can be enhanced.
Meanwhile, the risk of contact between metallic parts in gears is increasingly high as a result of a recent approach to enhancing fuel efficiency by the reduction of the stirring resistance of lubricants by lowering the viscosity of differential gear oils or transmission oils to below the conventional level. Thus, materials that are extremely stable to shear and do not decrease viscosity are desired.
Based on this demand for performance enhancement, with respect to the J306 classification of minimum viscosities after 20 hours of the CRC L-45-T-93 shear test, it has been gradually required to meet a new classification that defines minimum viscosities to be possessed by drive oils after the same test for 5 times as long as usual, namely, 100 hours.
Poly-α-olefins (PACs) are synthetic lubricants that are widely used in industry as lubricant base oils satisfying the above requirement. As described in, among others, Patent Documents 1 to 3, PAOs may be obtained by the oligomerization of higher α-olefins using acid catalysts.
As described in Patent Document 4, ethylene/α-olefin copolymers, similarly to PACs, are known to be employable as synthetic lubricants having excellent viscosity index, oxidation stability, shear stability and heat resistance.
Conventional methods for the production of ethylene/α-olefin copolymers used as synthetic lubricants involve vanadium catalysts including a vanadium compound and an organoaluminum compound as described in Patent Document 5 and Patent Document 6. The mainstream of ethylene/α-olefin copolymers produced by such methods is ethylene-propylene copolymers.
Methods using catalyst systems including a metallocene compound such as zirconocene and an organoaluminum oxy compound (aluminoxane) such as, among others, those described in Patent Document 7 and Patent Document 8 are known to produce copolymers with high polymerization activity. Patent Document 9 discloses a method for producing a synthetic lubricant including an ethylene/α-olefin copolymer produced by using a combination of a specific metallocene catalyst and an aluminoxane as a catalyst system.
In recent years, there has been an increasing trend in the demand for PACs, ethylene-propylene copolymers or the like, which are synthetic lubricant bases having excellent low-temperature viscosity characteristics, heat resistance and oxidation stability. From the points of view of higher fuel efficiency and energy saving, further improvements in viscosity index and low-temperature viscosity characteristics are desired.
To meet such demands, PAOs have been invented which are obtained by, among others, methods described in Patent Documents 10 to 13 using a catalyst system including a metallocene compound such as zirconocene and an organoaluminum oxy compound (aluminoxane).
It is known that the shear stability of lubricant compositions is dependent on the molecular weights of constituent components. That is, a lubricant composition which contains components having a higher molecular weight is more apt to decrease its viscosity when subjected to shear stress and the rate of this viscosity drop is correlated with the molecular weights of components present in the composition.
On the other hand, the incorporation of high-molecular weight components enhances the temperature viscosity characteristics and low-temperature viscosity characteristics of lubricant compositions. That is, while components such as PAOs and ethylene-propylene copolymers provide an enhancement in the temperature viscosity characteristics of lubricant compositions as their molecular weights are higher, there is a trade-off in that shear stability is decreased. In this regard, lubricants have room for improvement in terms of the satisfaction of shear stability and temperature viscosity characteristics at the same time.